In the technical field of photon detection, the so-called avalanche photodiodes operating in Geiger mode (GMAPs) are known, which enable detection of individual photons.
An avalanche photodiode operating in Geiger mode, also known as single-photon avalanche diode (SPAD), is formed by an avalanche photodiode APD. Hence, it comprises a junction made of semiconductor material, which has a breakdown voltage VB and is biased, in use, with reverse-biasing voltage VA higher in modulus than the breakdown voltage VB, which, as is known, depends upon the semiconductor material and upon the doping level of the least doped region of the junction itself. In this way, the junction has a particularly extensive depleted region, present inside which is a non-negligible electrical field. Hence, generation of a single electron-hole pair, caused by absorption within the depleted region of a photon impinging on the SPAD, may be sufficient to trigger an ionization process. This ionization process in turn causes an avalanche multiplication of the carriers, with gains of around 106, and consequent generation in a short time (a few hundreds of picoseconds) of the so-called avalanche current, or more precisely of a pulse of the avalanche current.
The avalanche current can be collected, typically by means of an external circuitry connected to the junction, for example by means of appropriate anode and cathode electrodes, and represents an output signal of the SPAD, which will be also referred to as “output current”. In practice, for each photon absorbed, a pulse of the output current of the SPAD is generated.
The fact that the reverse-biasing voltage VA is appreciably higher than the breakdown voltage VB causes the avalanche-ionization process, once triggered, to be self-sustaining Consequently, once triggered, the SPAD is no longer able to detect photons, with the consequence that, in the absence of appropriate remedies, the SPAD manages to detect arrival of a first photon, but not arrival of subsequent photons.
To be able to detect also the subsequent photons, it is necessary to quench the avalanche current generated within the SPAD, arresting the avalanche-ionization process, and in particular lowering, for a period of time known as “hold-off time”, the effective voltage Ve across the junction so as to inhibit the ionization process. For this purpose, use of so-called quenching circuits, whether of an active or passive type, is known. Next, the reverse-biasing voltage VA is restored in order to enable detection of a subsequent photon.
This being said, the timing of response of the SPAD, i.e., the time required for generating an output current pulse following upon absorption of a photon, is affected principally by four factors: the time for collecting the carriers within the depleted region, typically of the order of a few picoseconds per micron of depleted region; the time of propagation of the avalanche, i.e., the time required for the entire junction to be brought into the breakdown region, typically of the order of a few tens of picoseconds; the time of diffusion of the carriers generated in the non-depleted region of the junction through the non-depleted region itself, typically comprised between a few tens of picoseconds and a few nanoseconds; and the drift time proper, for collection of the carriers at the electrodes.
In connection with the time of diffusion of the carriers through the non-depleted region, it should be noted that not only both of the carriers of each electron-hole pair generated following upon absorption of a photon within the depleted region concur to the generation of the output current. In fact, given the reverse biasing of the junction, to the generation of the output current there concur also the minority carriers of the electron-hole pairs generated following upon absorption of a photon outside the depleted region, hence in a non-depleted, i.e., quasi neutral, region.
For example, assuming a junction of a PN type with the P region arranged, with respect to the direction of propagation of the photons, upstream of the N region, there may contribute to the output current both the electrons of the electron-hole pairs generated in the quasi-neutral portion of the P region of the junction (also known as “dead layer”) and the holes of the electron-hole pairs generated in the quasi-neutral portion of the N region of the junction (generally known as “epilayer”). The set of the portions of the SPAD in which generation of carriers can take place following upon absorption of photons is known in general as “active area”.
In practice, in the present description the term “minority carriers” is used to indicate carriers that are minority carriers in the point in which they are generated following upon absorption of a photon. For example, assuming again a region of a P type, an electron of an electron-hole pair generated following upon absorption of a photon in this P region is a minority carrier, whereas the corresponding hole is a majority carrier.
Likewise, assuming a region of an N type, a hole of an electron-hole pair generated following upon absorption of a photon in this N region is a minority carrier, whereas the corresponding electron is a majority carrier.
This said, the aforementioned minority carriers can cause generation of corresponding output current pulses, in the case where they manage to diffuse until they reach the depleted region, without first recombining.
However, even though also the minority carriers of the electron-hole pairs generated outside the depleted region can contribute to photon detection, they require, in order to be able to reach the depleted region, diffusion times that can range (according to the point of generation and the doping level) between a few tens of picoseconds and a few nanoseconds. Consequently the carriers generated in the avalanche events triggered by them can be collected at the anode and cathode electrodes with considerable delays. As a result, there is a deterioration of the response time of the SPAD. In particular, the so-called diffusion tails are generated in the output current.
Similar considerations may be made regarding a so-called SPAD array, and in particular, a so-called silicon photomultiplier (SiPM).
In detail, a SiPM is formed by an array of SPADs grown on one and the same substrate and provided with respective quenching resistors (for example, of a vertical type) integrated in the SPADs themselves, these quenching resistors being uncoupled from one another and independent. Moreover, the anode and cathode electrodes of all the SPADs are configured so that they can be connected to a single voltage generator. In other words, the anode electrodes of all the SPADs are multiplexed with one another; likewise, the cathode electrodes of all the SPADs are multiplexed with one another. Consequently, the SPADs of the SiPM can be biased at one and the same reverse-biasing voltage VA; moreover, the avalanche currents generated within them are multiplexed together so as to generate an output signal of the SiPM equal to the summation of the output signals of the SPADs.
In practice, the SiPM is a device having a wide area and a high gain, capable of supplying, on average, an electrical output signal (current) proportional to the number of photons that impinge upon the SiPM. However, SiPMs present the same drawbacks as the SPADs that form them.
Moreover, the SiPM, as on the other hand also a generic array of SPADs grown on one and the same substrate, and the anode and cathode electrodes of which are not multiplexed with one another, is affected by crosstalk.
In detail, given any SPAD, the corresponding operation is inevitably affected by charge carriers generated in surrounding SPADs and by photons generated by electroluminescence during processes of avalanche multiplication triggered in surrounding SPADs.
In greater detail, it may be noted that SPADs operating above the breakdown voltage emit secondary photons as a result of electroluminescence, on account of various mechanisms, such as for example intraband recombination. The secondary photons are emitted generally within a range of wavelengths comprised between 400 nm and 2 μm, with a probability of emission that depends upon the reverse-biasing voltage VA applied.
The secondary photons can propagate and subsequently be absorbed in the depleted regions of the junctions of SPADs different from the SPADs in which they have been generated, triggering avalanche events, in which case they give rise to the so-called “prompt crosstalk” phenomenon, which manifests itself on time scales of the order of a few tens of picoseconds.
It is likewise possible for the secondary photons to be absorbed within quasi-neutral regions of SPADs different from the SPADs in which they have been generated, triggering avalanche events in these photodiodes, in which case they give rise to the so-called “delayed crosstalk” phenomenon, which manifests itself on time scales of the order of a few nanoseconds. The different time scale with respect to prompt crosstalk is explained by the fact that, given a secondary photon emitted in a first SPAD, the absorption of this secondary photon within the quasi-neutral region of a second SPAD leads to generation of a pair of carriers, one of which can effectively trigger an avalanche process in the second SPAD, but only after reaching the depleted region of the second SPAD.
In particular, the secondary photons that most concur to the phenomenon of delayed crosstalk are the ones with wavelengths comprised between 700 nm and 1100 nm, because, in this range of wavelengths, the coefficient of absorption of silicon is particularly low, and consequently these secondary photons can cover long distances before being absorbed.
To the phenomenon of delayed crosstalk there also concur the charge carriers that, after being generated within the quasi-neutral regions of SPADs, diffuse until they reach the depleted regions of SPADs different from the SPADs in which they were generated. On the other hand, these carriers can likewise reach the depleted regions of the same SPADs in which they have been generated, in which case they give rise to the so-called “afterpulsing” phenomenon.
In practice, the crosstalk causes electro-optical coupling between the SPADs of the SiPMs. Consequently, the crosstalk increases the total noise of the SiPM, especially in the case where the SiPM has large dimensions and is subject to a high reverse-biasing voltage VA. As a result, the sensitivity of the SiPM is limited; moreover, the probability of saturating the SiPM increases, since a certain number of SPADs is turned on owing to crosstalk, even in the absence of an external luminous flux.
In order to limit the crosstalk, and in particular in order to limit prompt crosstalk, the technique is known of forming, within each SPAD, a trench filled with metal material, which delimits the active area, as described for example in United State Patent Application Publication No. 2009/0184384, the disclosure of which is incorporated by reference.
As described in the document European Patent No. 1755171, the disclosure of which is incorporated by reference, likewise is known the technique of forming grooves of a substantially triangular shape around the active area of each SPAD, these grooves being coated with metal material. Given a SPAD, the grooves that surround it absorb possible secondary photons emitted by the given SPAD in directions roughly parallel to the surface of the given SPAD exposed to the luminous flux. In this way, a reduction of prompt crosstalk is obtained.
Moreover, given a SPAD, and considering the quasi-neutral region of the lower portion of the junction of this SPAD, the technique is known of forming an additional junction within the SPAD, arranged underneath the main junction and reverse biased in order to prevent the minority carriers that are generated in this quasi-neutral region from reaching the junctions of surrounding SPADs, with consequent reduction of delayed crosstalk. An example of this technique is described in United States Patent Application Publication No. 2011/0241149, the disclosure of which is incorporated by reference. This technique thus envisages providing, for each SPAD, three electrical terminals in order to bias the main junction and the underlying additional junction correctly. This leads to an increase in the complexity and a reduction in the so-called fill factor of the SiPM since part of the surface exposed to the luminous flux is coated with two metal layers.
As regards in particular the reduction of the delayed crosstalk, likewise known are photomultipliers of the type described in the document PCT Application No. WO2011/132025, the disclosure of which is incorporated by reference. In detail, according to PCT Application No. WO2011/132025, each photodiode comprises a layer of amorphous silicon, which is arranged underneath the respective junction and has been created by means of ion bombardment of epitaxial silicon. In this way, each photodiode comprises a region with high efficiency of absorption of photons having wavelengths in the infrared. However, the creation of the amorphous layer by means of ion bombardment may cause an increase in the defectiveness within the active area.
In addition, as described in PCT Application No. WO2012/083983, the disclosure of which is incorporated by reference, techniques of treatment of the substrate are known that enable a reduction of the number of photons that are reflected on the bottom surface of each SPAD, with consequent reduction of delayed crosstalk. Also in this case, however, the active area can present a certain defectiveness. Moreover, the contribution to the delayed crosstalk due to carriers generated by absorption of the photon, before the latter reach the bottom surfaces of the SPADs, is not reduced.
There is a need in the art to provide an avalanche photodiode operating in Geiger mode that will enable the drawbacks of the known art to be at least partially overcome.